This invention relates to positive expulsion tanks such as are presently employed for carrying rocket engine thrust producing propellants in space vehicles, or the like; and more particularly to the sectional thickness variations of the sheet "liners" or "bladders" or rolling "diaphragms" such as are employed in such tanks as are shown for example in prior U.S. Pat. Nos. 3,275,193; 3,494,513; and co-assigned U.S. Pat. No. 4,129,025. It is the primary object of this invention to provide for this purpose an improved diaphragm which is of novel sectional thickness variation design; such as will in improved manner eliminate the tendencies of such diaphragms to locally wrinkle (or buckle) incidental to shape inversion when operating to expel the tank contents. A further object is to attain this improved capability without resorting to use of accessory roll guide and/or control devices such as are disclosed for example in U.S. Pat. Nos. 3,275,193 and 3,494,513.